Active Spam Killer

http://www.paganini.net/ask/
Active Spam Killer (ASK) confirms the sender's email address before actual delivery takes place. A "confirmation message" is automatically sent to all "unknown" users. Once the sender replies to that message (a simple reply will do), all future emails from that person are delivered immediately. You can also specify (regexp) addresses to be immediately accepted, rejected (with a nastygram) or ignored. The package also includes a utility to scan your old mailboxes and generate a list of emails to be accepted automatically.

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